Earlier studies have reported that acute exposure to alkaline pH strongly inhibits ammonia excretion in freshwater rainbow trout, but the Lahontan cutthroat trout thrives in Pyramid Lake, Nevada, at pH 9.4. We investigated the rates and mechanisms of ammonia and urea excretion in this species in Pyramid Lake water to determine whether special strategies are employed to excrete nitrogenous wastes in an environment unfavourable for ammonia excretion. The majority of nitrogen wastes (N-wastes) were excreted as ammonia (56 % through the gills, 10 % through the kidney), while urea excretion accounted for 34 % (32 % gills, 2 % kidney). Ammonia excretion was dependent on the NH3 partial pressure gradient (deltaPNH3) across the gills and independent of Na+ influx and acidification of the gill water boundary layer. Acute exposure to more alkaline water (pH 10) decreased ammonia excretion by 52 %, while exposure to neutral water (pH 7.6) increased ammonia excretion by 200 %. When fish were held in a ‘closed system’ for 8 h, ammonia excretion decreased as water ammonia levels increased over the first 6 h. However, after 6 h a marked increase in ammonia excretion occurred which may have been associated with an increase in the PNH3 gradient and/or activation of a carrier-mediated transporter. We conclude that Lahontan cutthroat trout, adapted to pH 9.4 water, maintain N-waste excretion by modifying mechanisms common to other teleosts. These modifications include lower rates of ammonia excretion, a higher ratio of urea excretion to ammonia excretion, a higher rate of renal ammonia excretion, greater plasma pH and greater total ammonia level (increased plasma PNH3), which facilitate the diffusive excretion of NH3 across the gills, and a lack of dependence of ammonia excretion on Na+ influx.

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